Permanent magnet stepper motors are currently used in a wide variety of apparatus including cameras, printers and scanners. Their ability to effect discrete and precise movement makes them the preferred choice for driving mechanical elements in this type of equipment.
Referring to FIG. 1, an exploded perspective view of a permanent magnet stepper motor of the type known in the prior art is shown. The stepper motor 10 includes a multi-pole permanent magnet rotor 11 carried by a shaft 12. The shaft 12 is mounted for rotation in front and back bearings 14 and 16 respectively. The bearings are mounted in front and back end plates 18 and 20. A pair of plastic bobbins 22 and 24 carry a pair of coils 26 and 28. A pair of soft magnetic metal stator yokes 30 and 32 each having a plurality of stator fingers 34, and a pair of soft magnetic stator shells 36 and 38, each having a corresponding plurality of stator fingers 40 are arranged such that the stator fingers 34 and 40 extend into the plastic bobbins in an interdigitated fashion to form stator poles when current is applied to the coils 26 and 28. The front and back end plates 18 and 20 are mounted on the outside ends of the stator shells 36 and 38, and the rotor 11 is supported by shaft 12 for rotation within the bobbins 22 and 24. The stator fingers in bobbin 22 are angularly offset from the fingers in bobbin 24 by one half the finger spacing so that current can be alternately applied to coils 26 and 28 to turn the rotor 10 in a stepwise fashion, thereby applying torque to shaft 12.
The manufacture and assembly of such stepper motors is presently a complex process requiring many steps and critical alignments, thereby making the motors relatively expensive. There is a need for an improved motor design and assembly method that is adapted to robotic automated assembly to reduce the cost and improve the reliability of the motors.